Thanks for all the suggestions. Just to close this off, I tried all the usual stuff - big screwdrivers, hide mallet, etc., and there were no other bolts holding it down that I hadn't already removed. Eventually I used a steel wedge, which I managed to tap into the joint such that it was actually splitting the gasket material (rather than bearing directly onto the metal). A few careful taps each side and it finally started to move. I didn't like having to resort to such methods but there's no damage, so it looks like I've got away with it.